What's a sensible sized alternator

[pagoda]

My fridge only draws somewhere just over 4 amps, no matter what its starting temperature. OK, it will run for longer but I think a 55 Amp alternator can cope with that.

Quite, I only run our fridge under engine or while on mains, but replacing most of your lamps with LED variants will cure much of your (OP's) power consumption. Do you not have any solar charger or wind-genny? Are four big lead batteries not slight overkill?

 

[pvb]

Not since it was fitted

The engine is a D1-30. 120A is what it comes with.

VP spec says 115A.

 

[prv]

VP spec says 115A.

Oops

I was sure it was 120A, but you may well be right. Still a welcome improvement over the car-sized units on older engines, and recognition that on a yacht the engine is almost as important for generation as for propulsion.

Pete

 

[William_H]

I am thinking of adding more battery capacity on the boat.

The idea is to have 340 A/h plus the 75 A/h engine start battery, with a splitter to manage the various 'needs'. And an Adverc or similar to help.

The suggestion is to have a higher output alternator as well, on a Yanmar 3YM20.

What would be a sensible output from the alternator?

Well you could go ahead and spoend a lot of money but I think there is a good case for just increasing battery capacity. The additional battery capcity will increse the current the total batteries take. Without resorting to an Adverc. If you fit an amp meter so you can monitor the actual currrent of the alternator (and battery charge) you might find that it is pretty reasonable. Even less concern if you runa shore power charger or solar top up.
There is no way around the loss of horsepower with charging. If I remember correctly 650watts ie 46 amps equals 1 Horsepower. That is before losses of heat etc. Perhaps 30 amps per horsepower in pracice. More current means more horsepower lost and also more demands on the belt drive system.
IMHO just watch an amp meter for a season with the added batteries and see if you have a problem. good luck olewill

 

[sailinglegend420]

The OP's post asked "What's a sensible sized alternator?" The answer should be "Best Practice" - if you can afford it.

Trojan recommend a changing current of Capacity (C)/10 So the Op's 340 AH bank would need to be charged at 34 amps. Charging at this rate maybe too slow. Other FLAs recommend C/5, say 70 amps, and AGMs could be Cx1.5 or more, which could be around 500 amps. It is recommended to add a larger alternator to also supply the house loads, so the Op's alternator should be sized at 340/4, so a 90 amp alternator would be good. This will give the fastest charge.

Many boats come with a cheap standard automotive alternator with an internal regulator that is only designed for charging a start battery that never gets highly discharged. This alternator regulator will not properly charge a deep cycle house bank, so the battery capacity and life will be severely reduced. Marine alternators will be much more expensive and can be controlled by an external multi stage regulator. They also may have a “Hot Rated” output that will give a more realistic idea of their true performance when under continuous heavy load.

Boats about 15 years old may have an automotive alternator which charges at only 13.8 – 14 volts. Battery charging at these voltages will take much longer. If split diodes are used to charge both the starter and house batteries then the voltage drop lost across these diodes (0.7volts) means that the batteries only get a charging voltage of 13.1-13.3 volts. Batteries may never get anywhere near properly charged at these voltages!

Fitting an external regulator to an alternator may be difficult and can involve opening the regulator to connect to the field wire. There are alternator to battery chargers which boosts the charging voltage without requiring any modification to the alternator. To boost the low output of a 13.8 volts alternator to 14.4 or higher puts a heavy load on what is already an old alternator. Better to buy a new marine alternator which can accept an external regulator.

Boats with a more modern 14.2-14.4 volt alternator with a built-in regulator may have a battery sense wire that can be connected directly to the battery to allow the alternator to measure the actual battery voltage and so compensate for split diodes and cable losses. They will also have a simple temperature regulator which will soon drop the output voltage down well below the 14.4v gassing voltage - it thinks the battery is close to it in the engine compartment. This shouldn't happen on a boat - batteries should be well away from the engine compartment, so they will see a lower voltage than they need once the alternator has warmed up, and so maybe undercharged.

There are many other very good reasons why you should fit an external regulator, even if your internal regulator is producing 14.4 volts.

1. A good external regulator will be a multi-stage device that can raise the alternator output voltage and charge the battery faster, and reduce the output when the battery is fully charged to prolong the battery life.

2. It should have settings for different battery types, but it should also be programmable to match the alternator and battery bank sizes. Balmar's regulators allow many parameters to be changed, for example they may set the Boost voltage to 14.6, hold that for 45 minutes and then reduce it to 14.4 for the duration of the absorption stage. They can be programmed to stay longer in the absorption stage without dropping down to float too early.

With sealed batteries all battery manufacturers recommend all charging sources which will drop the charge down to float mode at 13.2-13.8 volts to avoid gassing and overcharging.

3. It will delay the charge current for about a minute at start up until the engine oil is well distributed. A 100 amp alternator draws 4 HP which is a heavy load on an engine when starting with a weak battery.

4. It will also accept a sensor to measure the alternator temperature and control the charge rate. Even a 100 amp alternator could be providing 15 amps to the boat systems, another 25 amps to say a watermaker or an inverter, and then a heavily discharged bank may be demanding 50 or 60 amps. In this situation the batteries may not get charged and the alternator will burn out from continually trying to deliver its maximum output if not protected by a temperature sensor. It is often too easy for the alternator on a boat to get too hot if used for long periods.

5. It will also accept a sensor to measure the battery temperature which will rise with a heavy charge current. At 25ºC batteries start to gas at 14.4v, at 40ºC they gas at 14v so the external regulator will reduce the charging voltage automatically to compensate for this. If batteries are fitted in an engine compartment it is very easy for them to get too hot and lose water. This is fatal for sealed batteries. Sailing in high temperature regions may mean the batteries are already at 35C before charging starts.

6. It will have a voltage sensor at the battery not on the alternator. This will compensate for split diodes or losses on cable runs to the battery.

7. A Balmar regulator has a “small engine mode” that allows you to cut the output to 50% with a switch to reduce the alternator load on the engine when you suddenly need extra power in a big sea. It also has belt manager to reduce the load on the alternator. A 100 amp alternator set to 70% will run cooler and produce 70 amps at a lower speed.

 

[vyv_cox]

Are four big lead batteries not slight overkill?

We found the improvement obtained by increasing domestic battery capacity from 220 Ah to 330 Ah to be well worth the expense and rearrangement that was needed. At anchor and running the fridge 24 hours per day we would find battery voltage at 12.0 or occasionally less first thing in the morning, whereas after the upgrade it is rarely less than 12.4. The reduced daily voltage fluctuation will undoubtedly lengthen the life of the batteries and pay for itself. Many liveaboards have far more battery capacity than we do, maybe reflecting additional requirements and use throughout the year.

 

[pvb]

We found the improvement obtained by increasing domestic battery capacity from 220 Ah to 330 Ah to be well worth the expense and rearrangement that was needed. At anchor and running the fridge 24 hours per day we would find battery voltage at 12.0 or occasionally less first thing in the morning, whereas after the upgrade it is rarely less than 12.4. The reduced daily voltage fluctuation will undoubtedly lengthen the life of the batteries and pay for itself. Many liveaboards have far more battery capacity than we do, maybe reflecting additional requirements and use throughout the year.

Increasing the domestic battery capacity is one of the most cost-effective improvements. Longer intervals between charging, faster charging, longer battery life.
.